Sputter coating is a process carried out in a vacuum chamber, typically filled with either a chemically inert gas or a reactive gas, in which a substrate to be coated is mounted facing a target formed of the coating material. In the chamber, the target is subjected to an electrical potential negative with respect to the chamber wall or some other anode within the chamber to produce a potential gradient that causes electrons to be emitted from the target and attracted toward the chamber anode. The emitted electrons move toward the anode, and while moving strike and ionize atoms of the inert gas by stripping electrons from them. Positive ions of the gas are thereby formed and attracted toward the negatively charged target which they strike, transferring momentum to the target material, and ejecting particles of the material from the target surface. The substrate to be coated, which is positioned in the chamber usually with its surface facing the target so as to intercept the moving particles of coating material sputtered from the target, receives some of the ejected particles, which adhere to and coat the substrate surface.
In magnetron enhanced sputtering processes, a magnetic field is formed over the target surface with the magnetic field having component lines extending parallel to the target surface. In many applications, the field lines arch over the target surface and form a closed magnetic tunnel. The magnetic field causes the electrons moving from the target to curve in spiral paths over regions of the target surface enclosed by the field, thereby increasing the electron density in the enclosed space, and resulting in an increase in the rate of electron collisions with gas atoms over the enclosed regions of the target surface. The increased collision rate in turn increases the ionization of the gas in the enclosed space and thus increases the efficiency of the sputtering process at the underlying target region. Where the magnetic field is sufficiently strong over the target surface, a glowing ion cloud or plasma is seen trapped within the field over the region of the target surface
In the commonly assigned and copending U.S. patent application Ser. No. 07/339,308, filed Apr. 17, 1989, now U.S. Pat. No. 4,957,605, entitled "Method and Apparatus for Sputter Coating Stepped Wafers," expressly incorporated herein by reference, a sputter coating apparatus and method are disclosed in which a single-piece concave annular target is provided with a pair of concentric annular electromagnets having concentric annular pole pieces behind and at the rim of the target. The fields produced by these magnets cause the formation of a pair of concentric plasma rings overlying concentric sputtering regions on the target surface. The two plasma rings are alternately energized by alternately supplying current to the magnet coils while the target power is switched between two controlled power levels in synchronization with the switching of the current to the magnetic coils. This causes the two target regions to be alternately activated so that the sputtering from the regions is alternately switched on and off. This switching provides different controllable rates of sputtering from inner and outer concentric regions of the surface of the sputtering target.
Separate control of the sputtering from the plural target regions enables the control of the distribution characteristics of the sputtered material deposited on the surface of the substrate or wafer being coated. A varying of the relative parameters affecting the energization of the two target regions, as for example the "on" power levels or the relative duty cycles of the activation of the target regions, provides control of coating uniformity on the substrate surfaces. This control is especially important where differently facing surfaces of substrates, such as steps on the surfaces of semiconductor wafers, must be uniformly coated. The aforereferenced U.S. Pat. No. 4,957,605 particularly describes in detail certain effects on the coating uniformity caused by target and substrate geometry and by the electrical parameters relating to the energization of the target and the plasmas.
By its very nature, cathode sputter coating involves the removal of material from the sputtering target and the deposition of the sputtered material onto the substrate surface. The removal of material from the cathode sputtering target consumes the target, changing the shape of the target surface and reducing the thickness of the target until eventually an erosion groove extends to the back surface of the target. The erosion of the target surface is usually uneven, being concentrated in areas which underlie the denser regions of ion concentration or plasmas in the space above the target adjacent the target surface. The formation of this erosion groove alters the performance of the sputtering target, generally with a declining emission rate from the sputtering target region, a phenomenon referred to as rate "roll-off". Compensation for the effect of a declining deposition rate is usually achieved by progressively increasing the power applied to the target over the course of the useful target life in order to maintain an acceptable or even constant deposition rate onto the substrates.
With magnetron sputtering devices the plasmas are generally confined to one or more regions of a target surface, in part due to design requirements of the magnet structure, and in part due to certain performance requirements which necessitate the location of the plasmas in specific geometric positions in relation to the substrate surfaces to achieve a desired coating distribution on the substrate. Because the positions of the plasmas determine the locations from which the sputtering material is emitted, which determines the corresponding distribution of the deposited coating material on the substrate surface, movement of the plasmas, while smoothing target erosion, can interfere with the achievement of coating uniformity. Precise differential control of sputtering rates from different sputtering regions on a sputtering target is important in achieving the uniformity of the deposited coating on the substrate. It is not only target erosion that affects the uniformity of coatings on substrates, but pressures, temperatures, gas compositions, material properties and property uniformity of targets and substrates, device configurations on the wafer surfaces and process step sequences which require variations in process parameters to achieve high coating quality uniform coatings. Not all factors which affect the coatings are easily or accurately predictable.
In bias sputtering, a voltage which is negative, but less negative than that imposed on the target, is applied to the substrate being coated. This bias voltage causes a certain amount of "back sputtering," or sputtering from the coating which has been deposited on the substrate surface, due to the impingement of ions produced by electrons emitted from the substrate. In some, but not all, sputter coating processes, bias sputtering is desirable to impart certain properties to substrate surface.
The complete processing of a semiconductor wafer involves a number of subprocess steps, not all of which are sputter coating processes. Semiconductor wafers are manufactured by a sequence of processes by which layers of conductive or insulative material are selectively deposited onto and removed from the surface of a wafer substrate. Removal of layers or portions of layers may be by a sputter etching process, a process in which the substrate is biased without the presence of a sputtering target so that ions bombard the substrate surface to remove material therefrom.
Both sputter coating and sputter etching processes may take place in the presence of an inert gas such as argon or the presence of a reactive gas such as oxygen, chlorine, or nitrous oxide. A reactive gas may facilitate the deposition or removal of material from the wafer or may combine with the surface material or otherwise impart certain desirable properties to the wafer being processed.
Each of the processes performed on semiconductor wafers requires the control of not only the applicable electrical parameters such as target and substrate voltage, current and power, magnetic field placement and strength, and the timing of the application of the electrical parameters, but also of the pressures and flows of various gasses, the maintenance of temperatures of targets, wafers and gasses, and other factors which affect the process.
In addition, machines for processing the wafers are designed in many different forms. In sputter processing machines such as those described in U.S. Pat. Nos. 4,909,695 and 4,915,564, for example, both assigned to the assignee of the present application and both hereby expressly incorporated herein by reference, methods and apparatus for sequentially performing a plurality of different processes on wafers in a single main vacuum chamber, having therein separately isolatable process chambers, are disclosed. At each of the chambers, in the apparatus of the above incorporated patents, up to five different wafers can be processed simultaneously, one at each of the chambers, and each through five processing steps performed in different chambers. Between the performance of each of the processing steps, the chambers are opened, the wafers are moved from chamber to chamber, wafers are cycled into and out of the machine through a loadlock, the chambers are resealed and the atmospheres in them returned to the controlled environments needed for the processes performed in them. All of the steps and substeps in the machine sequences must be controlled in synchronism with the control of the steps of the various processes.
Wafer processing machines have been provided with controls which regulate or vary process parameters or maintain parameters at certain setpoints. The sequencing of process steps and the movement of wafers through the apparatus are also controlled. Some of the machine controls have employed closed loop feedback control of certain parameters or programmed control of process sequences.
Over the course of running processes on wafers, however, the need to change parameter setpoints, modify program steps or sequences, and otherwise alter the course of machine operation is often desirable. The ability of the machines to repeat processes and maintain parameter control automatically does, in many cases, improve the efficiency and quality control of the performed processes. The need for such changes, however, is not always predictable and the responses to observable performance deviations is not always capable of control by control algorithms programmed into the machines. Human operator intervention and human decision making in the modification of the machine process setpoints and control programs is, in many cases, necessary. The speed and efficiency with which such changes can be made often determines the ultimate quality and efficiency which is attainable with the equipment. Machines of the prior art have not adequately provided for automated control which is sufficiently flexible to allow for the optimum input from the operator.
Accordingly, there is still an unfulfilled need in the prior art for effective interface between automated wafer process control and user decision making.